1. Field of the Invention
The present invention relates to an image display apparatus capable of displaying an image on a given plane of projection by beam raster scanning.
2. Description of the Background Art
Optical scanners deflecting and scanning light beams such as laser beams are utilized for optical equipment such as a barcode reader, a laser printer, and a display. Some such optical scanners include a polygon mirror scanning reflected light by rotation of a polygonal shaped mirror with a motor, a galvano-mirror causing a plane mirror to rotate and oscillate with an electromagnetic actuator, or the like. Such optical scanners require mechanical drive mechanisms for driving a mirror with a motor or with an electromagnetic actuator; however, such drive mechanisms are relatively large in size and expensive, inhibiting miniaturization of the optical scanners and increasing the cost thereof.
For miniaturization, cost reduction, and improved productivity of optical scanners, development of micro optical scanners where components such as mirrors and elastic beams are molded in one piece is now in progress using micromachining technologies for micromachining silicon or glass with application of semiconductor manufacturing technologies.
There is an image display apparatus which is provided with two such optical scanners as described above and displays a two-dimensional image on a plane of projection by raster scanning of light beams reflected off mirrors of those scanners.
This image display apparatus can provide a stable image display by bringing an image signal of a displayed image into synchronization with raster scanning (horizontal and vertical scanning). However in horizontal scanning where in general a mirror makes resonant oscillations for a horizontal scan output of greater amplitude, it is not easy to match the resonant frequency (horizontal scanning frequency) of the resonant oscillations and the horizontal synchronizing frequency of the image signal for synchronization. Some well-known techniques for synchronization are described in the following paragraphs (i), (ii), and (iii).
(i) In the designing or manufacturing stage of an optical scanner, the resonant frequency is made approximately equal to the horizontal synchronizing frequency of an image signal.
(ii) An image frame (image signal) is temporarily stored in a buffer memory and a readout clock is generated, using a PLL circuit, from a drive signal for horizontal scanning including resonant oscillations, based on which clock an image signal for each horizontal line is read out from the above-described buffer memory (see for example Japanese Patent Application Laid-open No. JP11-146222).
(iii) An optical scanner is used, which has a configuration capable of controlling the resonant frequency. For example as in the technique disclosed in Published Japanese Translation of PCT International Application No. 2004-518992, an electric field is applied to a flexible arm extending from an vibrator (mirror), causing a bending of the arm and thereby changing the moment of inertia of the vibrator, for resonant frequency control.
The technique described in the above paragraph (i), however, may cause substantial changes in the output of horizontal scanning (horizontal scan width) with fluctuations in the resonant frequency due to environmental changes such as temperature or changes with time, thereby having difficulty in providing a stable image display by beam raster scanning accompanied with resonant oscillations in the horizontal scanning direction (the main scanning direction).
The technique described in the above paragraph (ii) requires an additional circuit such as the PLL circuit described above, thereby complicating the device configuration.
The technique described in the above paragraph (iii), although capable of controlling the resonant frequency, necessitates a complicated structure for that control.